Last year, astudyby University of Queensland climate scientist and founder of
the Skeptical Science website, John Cook, revisited consensus on
climate change within the scientific literature.

The researchers examined 12,000 studies. Similar
to other studies, Cook et al.concludedthat 97 per cent of the papers
that expressed a position on the causes of climate change pointed
to human activity as the main driver.

The study received a lot of media coverage at
the time,ranking
11thamong new science papers receiving the most
attention online in 2012, and even earning a mention from the US
president. But it's alsoprompted some heated
discussion.

Most recently, economics professor Richard Tol
published acritiqueof the paper in the journal Energy
Policy. Tol takes no issue with the paper's conclusion about an
overwhelming scientific consensus in the literature. Instead, the
crux of his argument is with specifics of the
methodology.

Consensus is complicated. And reducing a complex
question to a simple number is going to be fraught. So why do it?
We asked climate scientists what consensus means to them, if it can
be measured and how they use consensus in their own
work.

Why measure consensus?

Climate scientists overwhelmingly agree that
recent warming is being driven by human activity rather than by
naturally occurring processes. That's typically what people mean
when they talk about a 'scientific consensus' on climate
change.

The consensus is so strong because scientists
can be sure of the fundamental physical principles underpinning
greenhouse gas warming. Indeed,journalist Seth
Borenstein recently quoted a scientistequatingthe strength of the evidence with
that linking smoking with lung cancer.

However, studies have shown that outside
scientific circles, a large proportion of the general public is not
aware such overwhelming agreement exists among climate scientists.
A recent US poll suggested justone in tenpeople put the figure at more
than 90 per cent.

Accordingto the authors, the motivation for
the Skeptical Science paper is thatstudiessuggest people are more likely to
accept human-caused climate change - andaction to tackle it- once the broad
agreement between scientists is highlighted. Richard Allan,
professor of climate science at the University of Reading, explains
how the study may have helped connect the dots in public
minds:

"[T]he
analysis of Cook et al. was probably useful in dispelling the myth
sometimes portrayed by the media that there is substantial debate
over the robust scientific evidence relating human activity to
current and future climate change, which there isn't."

Communicating consensus

Others argue there's only so far this approach
can go towards correcting the mistaken impression in the public
consciousness that there's disagreement among scientists - referred
to as the "consensus gap".

Dan Kahan, professor of psychology at Yale Law
Schoolexplainsa concept known as "motivated
reasoning" - where peopleselectively use or dismiss
evidence depending on whether it is consistent or inconsistent with
theirworldviews or cultural bias.Release as many statements of consensus as you like, Kahan
contests, you're justpreaching to the choir.

The value of consensus can still be communicated
effectively, argues Dr Adam Corner, psychologist at Cardiff
University, in a recentGuardianarticle:

"The challenge is not to
find a non-political way of engaging people on climate change, but
to embrace the fact that politics permeates the discourse on
climate change … pointing to a row of nodding scientists and
expecting this to catalyse public concern is not going to get us
far - no matter what the 'magic number' attached to the consensus
is."

Another point worth noting is that in pursuit of
a single number to capture consensus, the answer you get depends on
the question you ask, explains Professor Kevin Trenberth, climate
variability expert at the US National Centre for Atmospheric
Research:

"Very small
changes can change the perceived consensus easily. 97% of
scientists may agree that global warming from humans is happening
but add any qualification like its timing or magnitude and it would
change."

And this can be unhelpful if clear communication
is the objective. Arguing over what the precise number should be
runs the risk of giving the impression there's more uncertainty
among scientists than there is.As long as there
appear outwardly to be arguments over consensus, it's easy for
those who wish to discredit the science to avoid engaging with the
level of agreement that really exists among scientists.

Nuances aside, the scientists we spoke to
generally tended to see the value of emphasising the broad strength
of consensus for communication purposes.Andrew
Dessler, professor of atmospheric sciences at Texas A&M
University, explains:

"After all, if you're
looking for an expert medical opinion, and you find out that 97 per
cent of the specialists agree about the course of treatment, you
can be justly confident that that's the best advice that medicine
can give you".

But to the climate science community, the
paper's conclusions come as no surprise. Independent climate
researcher professor James Annan explains:

"I don't
think the Cook et al paper really told us anything we didn't
already know (where "we" here is the vast majority of climate
scientists), and as such it wasn't really much of a scientific
contribution. That's not to say it wasn't worth doing, but it was
clearly playing more in a political than scientific
sphere."

"The only
time I hear about "consensus" is in the public debate over climate.
Scientists never ever talk about consensus - by going to meetings
and reading the peer-reviewed literature, you can figure out what
your colleagues think".

Consensus: a step on the road to scientific
progress

Though it may not be much of a conversation
topic among scientists, consensus as a concept has a fundamental
place in driving scientific progress. Dessler continues:

"For each
question that arises (e.g., is the earth warming?), there comes a
time when the evidence is so overwhelming that the experts
independently realize that the problem is essentially solved … At
that point consensus exists on the answer and the question is no
longer interesting - and scientists move on to the next interesting
question".

So it's important to understand consensus as the
product of a body of research rather than the starting point, Annan
explains:

"[W]e don't do research
by agreeing a consensus - rather, it is what emerges where we can
no longer find arguments."

Arriving at a consensus - even if it's an an
unspoken one - drives scientific progress by taking research in new
directions, Dessler adds:

"Consensus
determines what we know, and it also determines what we don't know.
[It] also determines what the interesting questions are, which in
turn determines what people work on … [T]he question of whether the
earth has warmed over the last century is not an interesting
question. We have firmly established that has. So people don't
spend much time working on that."

On the other hand, scientists are working very
hard tounderstandto why surface temperatures
haven't risen as quickly in the last decade and a half compared to
previous ones - a far more interesting topic, says
Dessler.

A growing body of research suggests the surface
warming slowdowncan be explainedby a strengthening of
Pacific trade winds, causing more heat tofind its wayto thedeep oceanrather than staying at the
surface. Such natural variation is by its nature short-term, and
scientists expect the long term warming trend to
continue.

How a scientific idea evolves

The very early stages of exploring a new topic
in any area of science is the evolution of a hypothesis to a
theory. This happens as evidence builds up to either support or
refute it.

Map of Arctic sea ice extent in 2012 compared with
1979. Purple line shows 1979-2012 long term average. Source:
National Snow and Ice Data Centre (NSIDC) See full
video here

A good example of a new and still very
speculative field is whether diminishing Arctic sea ice is linked
to Northern Hemisphere extreme weather. Professor Jennifer Francis,
whoseworkproposing a connection prompted most of
the research in this field, explains:

"Consensus
comes in wherever there isn't absolute proof of a theory, which is
most of the time. In my work regarding links between rapid Arctic
warming and changes in weather patterns, I would not say we have a
consensus yet. When you do achieve consensus, it's when a
hypothesis transitions to a theory … This is a key distinction in
science."

In other words, broad agreement on certain
fundamentals of climate science can coexist alongside less
agreement in other areas. A very clear illustration of this is the
recentreportby the UK's Royal Society and the US
National Academy of Sciences, whichlays outwhat scientists know about climate
change, where consensus is growing, and where there is still
uncertainty.

Consensus comes in many guises

There's no doubt that consensus as applied in
the scientific world is a complicated concept. Each time consensus
pops up in the media, it raises interesting questions about how to
measure it and what expressions of consensus mean in different
scientific, social and political contexts.

However we choose to talk about consensus, all
we're really doing is exploring how to present scientific facts to
different audiences. The facts themselves don't change. That an
overwhelming consensus exists among scientists about human-caused
climate change isn't in doubt. But then again, it never
was.